Ultra high input impedance/voltage range amplifier

ABSTRACT

An ultra high input impedance and input voltage range amplifier wherein a low voltage input stage is cascaded with an emitter follower transistor stage which provides an excess current sink to keep the voltage across the input stage at a low value.

BACKGROUND OF THE INVENTION

The present invention relates to solid state amplifiers, and moreparticularly to an amplifier circuit having a high input impedance andhigh input voltage range.

In measuring circuits, it is sometimes necessary to present a very highinput impedance to the circuit being measured so as not to affect itsperformance. Also buffer amplifiers are required to provide isolationbetween a high impedance transducer, such as a condenser microphone oran electrostatic accelerometer, and its load. Such high impedancerequirements are typically limited to insulated gate field effecttransistors (IGFET) and junction effect transistors (JET) as sensingdevices, replacing the previous vacuum tube cathode follower andtransistor emitter follower circuits.

For ultra low leakage current applications, the IGFET is the mostdesirable choice. Since these devices are usually limited to 15-30 voltpower supply voltages, maximum, a usual measurement limitation is toimpose a few dozen volts working voltage range. Thus, for applicationswhere a measurement range of hundreds or thousands of volts is needed,these devices are not suitable.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a high impedance amplifierwith specifically a high input voltage range which uses a low voltage,low leakage current input sensing stage. In one embodiment, the sensingstage is provided with a wide tolerance constant current source and iscascaded with an emitter follower transistor having a Zener diode in theemitter circuit to limit the voltage that the input stage "sees" to areasonable and safe value. The input stage may be an IGFET in a sourcefollower configuration, or for optimum performance, an operationalamplifier in a voltage follower configuration.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of one embodiment of the presentinvention; and

FIG. 2 is a schematic diagram of a second embodiment of the presentinvention which uses an operational amplifier.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1. Q₁ is field effect transistor configured as asource follower. An input voltage, e_(in), is applied to the gate of Q₁and a scaled down output voltage, e_(out), is taken from the junction ofresistors R₂ and R₃ in the Q₁ source circuit. R₂ and R₃ are connected asa voltage divider network between the Q₁ source and negative terminal,-V, of a power supply.

A PNP transistor, Q₂, collector is connected to the Q₁ drain and emitterconnected through resistor R₁ to the positive terminal, +V, of the powersupply. R₁ establishes the Q₂ collector current. Bias for Q₂ is providedby the voltage divider network of R₄ and R₅ connected between -V and +V,with the Q₂ base connected to the junction of R₄ and R₅.

A PNP transistor Q₃ is cascaded with Q₁ and is configured as an emitterfollower, to provide an "excess current sink" for Q₂ with the Q₃ baseconnected to the Q₁ source and the Q₃ collector connected to -V. The Q₃emitter is connected to the Q₁ drain through a reverse biased Zenerdiode D₁.

The power supply differential voltage is high enough so that the JET Q₁operates linearly for all anticipated input voltage values. The Zenerdiode D₁ establishes the voltage, V_(ps), across Q₁, which is generallyof the order of 5 to 15 volts. Transistors, Q₂ and Q₃ are high voltagebipolar transistors since they will "see" to within a few volts of thefull power supply voltage from -V to +V. Thus, Q₁ never has to "see" anyvoltage greater than approximately a dozen volts because Q₂, Q₃, R₁, R₂,and R₃ drop virtually all of the high voltage of the power supply.

FIG. 2 illustrates how a commercially available operational amplifier(OP AMP) can be used as a basic circuit building block, allowing anormally low voltage OP AMP to be chosen strictly on the merit of lowinput leakage current and offset voltage. The basic circuit desired is avoltage follower to provide high input impedance.

The positive voltage terminal, +V_(cc), of an OP AMP is connectedthrough a PNP transistor Q₁₁ to the positive terminal, +V, of a powersupply. Q₁₁ is configured as a current source with its collectorconnected to +V_(cc) and its emitter connected through currentdetermining resistor R₁₁ to +V. Likewise, the negative voltage terminal,-V_(cc), of the OP AMP is connected through a PNP transistor Q₂₁,configured as a current source with resistor R₂ as the currentdetermining resistor, to the negative terminal, -V, of the power supply.

PNP transistors Q₃₁ and Q₄₁ are connected to the output of the OP AMPand are configured as emitter followers. The collectors of Q₃₁ and Q₄₁are connected to -V and +V, respectively, and the respective emittersare connected through reverse biased Zener diodes D₁₁ and D₂₁ to +V_(cc)and -V_(cc) respectively.

A voltage divider network of resistors R₃₁, R₄₁ and R₅₁ between +V and-V provide bias voltages to the Q₁₁ and Q₂₁ bases at the R₃₁ and R₅₁junction and the R₄₁ and R₅₁ junction, respectively. An input voltage,e_(in), is applied to the non-inverting input of the OP AMP, and anoutput voltage, e_(out), is obtained at the output of the OP AMP withunity gain.

The OP AMP is effectively "floated" to be used over very large (hundredsor thousands of volts) voltage ranges. It not only provides a unity gainbuffer with high input impedance, but it also controls the verycircuitry which regulates where the "float point" voltage is.

The transistors Q₁₁, Q₂₁, Q₃₁ and Q₄₁ are high voltage bipolartransistors capable of operating at the full power supply differentialvoltage, but the Zener diodes D₁₁ and D₂₁ and the output connectedtransistors Q₃₁ and Q₄₁ limit the OP AMP voltages +V_(cc) and -V_(cc) toa reasonable and safe value by passing all current not needed by the OPAMP to the appropriate power supply terminal. The power supply voltageis great enough to allow proper current source and OP AMP operation overthe entire input-output voltage operating range; usually the supplydifferential is of the order of 2 to 18 volts greater than the e_(in)operating range, depending on the exact circuit characteristics desiredand chosen.

The circuit incorporating the operational amplifier has severaladvantages over the first embodiment, among which are: circuitsimplicity; large current source tolerance, thereby permitting the useof low cost ultra high voltage components; arbitrarily low input offsetcurrents may easily be obtained through proper choice of OP AMPs; andarbitrarily low distortion may be obtained with accuracies to 0.001% orbetter because of the large amount of positive feed forward and equallylarge feedback available.

Also, note that a voltage divider between E_(out) and -V (or any otherterminal voltage for that matter) may be added as is done with R₂ and R₃in FIG. 1 to provide scaled down output voltages.

What is claimed is:
 1. A high impedance amplifier comprising:a fieldeffect transistor connected as a source follower and having a highimpedance input, a source, and a drain; a current source including atransistor directly connected in a circuit with the drain of said fieldeffect transistor; a Zener diode directly connected across the sourceand drain of said field effect transistor and being reverse biased tomaintain a predetermined voltage thereacross; a current sink including atransistor having a collector, an emitter directly connected to saidZener diode, and a base directly connected to the souce of said fieldeffect transistor; and impedance means directly connected between saidsource of said field effect transistor and the collector of thetransistor current sink for providing an impedance matched outputtherefor, whereby said high impedance amplifier is protected from highvoltage transents appearing at the high impedance input of said fieldeffect transistor.
 2. A high input impedance amplifier comprising:anoperational amplifier having two input terminals, positive and negativevoltage supply terminals, and an output terminal for producing anelectrical output which is a function of the signal difference appearingat the input terminals; a source of positive voltage; a source ofnegative voltage; a first current source including a first transistorhaving a collector directly connected to said positive voltage supplyterminal of said operational amplifier and having an emitter connectedto said source of positive voltage; a second current source including asecond transistor having a collector directly connected to said negativevoltage supply terminal of said operational amplifier and having anemitter connected to said source of negative voltage; a first Zenerdiode having two terminals the first of which is directly connected tosaid positive voltage supply terminal of said operational amplifier; asecond Zener diode having two terminals the first of which is connectedto said negative voltage supply terminal of said operational amplifier;a third transistor having a base directly connected to said outputterminal of said operational amplifier, a collector directly connectedto said source of negative voltage, and an emitter directly connected tothe second terminal of said first Zener diode; a fourth transistorhaving a base directly connected to said output terminal of saidoperational amplifier, a collector directly connected to said source ofpositive voltage, and an emitter directly connected to the secondterminal of said first Zener diode, whereby said operational amplifieris protected from transiant voltage transents in excess of its ratedinput appearing across the two input terminals.